Fax machines may be interconnected using conventional analog telephone circuits operating end-to-end using G.711 signal encoding, which is also referred to as pulse code modulation. Fax messages may also be transmitted over packet networks using the T.38 protocol, which is also referred to as “fax relay”. When fax machines are interconnected via a packet network, G.711 encoding may be used during the setup of the connection.
Subsequently, when the actual fax transmission begins, the network connection may switch to fax relay operation. A newer version of the T.38 fax relay specification, hereinafter “enhanced T.38”, includes support for Super Group 3 (“SG3”) fax procedures. An older version of the T.38 fax relay specification, hereinafter “unenhanced T.38”, supports Group 3 (“G3”) fax procedures and does not support SG3 fax procedures. When SG3 fax machines attempt to communicate through an unenhanced T.38 fax relay link, the fax transmission will fail.